A Danish tourist was gang-raped near a popular shopping area in New Delhi after she stopped to ask for directions, police said.
It is the latest case to focus attention on the scourge of violence against women in India.
The 51-year-old woman was also robbed and beaten in the attack, which happened on Tuesday afternoon near Connaught Place.
The woman had lost her way and was trying to get back to her hotel, said a police spokesman.
She approached a group of men for directions, but they lured her to a secluded area where they raped her at knife-point.
The woman managed to reach her hotel and the owner called police. Several suspects have been questioned, but no arrests have been made.
"When she came, it was miserable," said Amit Bahl, owner of the Amax Hotel in the Paharganj area, which is popular with backpackers. The woman was crying and "not in good shape".
"I am really ashamed that this happened," said Bahl, who sounded shaken.
The problem of sexual violence in India has gained widespread attention since the gang-rape and murder of a young woman on a bus in December 2012.
Public fury over the case has led to more stringent laws that doubled prison terms for rape to 20 years and criminalised voyeurism and stalking.
But for many women, particularly the poor, daily indignities and abuse continue unabated and the new laws have not made the streets any safer.
Ranjana Kumari, director of India's Centre for Social Research, said the country's conservative, patriarchal traditions led men to use rape as a tool to instill fear in women.
"This mindset is not changing," she said. "It's a huge challenge."
Experts say the rapid growth of India's cities and the yawning gulf between rich and poor are exacerbating the problem of sexual violence.
Cultural stigmas, police apathy and judicial incompetence have long made it difficult for women to even report rapes.
Still, there has been a surge in the number of rapes being reported recently, suggesting women are emboldened to speak up.
Between January and October last year, 1,330 rapes were reported in Delhi and its suburbs, compared with 706 for all of 2012, according to government figures.